The present invention relates to a pattern forming method for forming a predetermined pattern by a photolithography method on a plastic substrate which composes a display device such as a liquid crystal display device, an organic EL display device, an electrophoresis display device, etc., and a display device manufactured using the same.
When a predetermined pattern is formed by a photolithography method on a substrate which composes a display device, the pattern is often required to be formed with high accuracy of dimension and position. For example, in a case of a pair of substrates which compose a liquid crystal display device, a transparent conductive film composed of Indium Tin Oxide (ITO) having a predetermined pattern is formed on each of the substrates, and then the pair of substrates are pasted with each other so that the both patterns face one another with predetermined accuracy of superposition. Thus, the predetermined patterns that are respectively formed on the substrates are required to be formed within a relatively constant range of dimensional accuracy.
Further, a color filter used for a liquid crystal display device is manufactured in such a manner that a plurality of patterns including a colored light-transmitting pattern of light""s three primary colors such as RGB, a light-shielding pattern such as a black matrix, etc. are sequentially formed. Thus, the respective patterns are required to be formed within a constant range of accuracy of superposition. The accuracy of superposition here is mainly achieved by relative accuracy of dimension and position among the respective formed patterns. The higher definition of a manufactured color filter pattern requires higher accuracy of superposition, and thus requires higher accuracy of dimension and position for each of the formed patterns. Further, a transparent electrode for liquid crystal driving composed of ITO is required to be formed on the color filter with the same accuracy.
For example, when the formed pattern of the color filter and the formed pattern of the ITO for liquid crystal driving are both formed with high accuracy of dimension and position, the both patterns are formed with fine accuracy of superposition over an entire substrate, so that each of driving ITOs 101 corresponds to one color of color filters Rxe2x80xa2Gxe2x80xa2B, as shown in FIG. 11(a). In contrast, when the accuracy of dimension and position is not good, positional misalignment occurs. When the accuracy of superposition of the both patterns is not good, namely an amount of the positional misalignment exceeds a tolerance, a so-called color mixture defect occurs, in which one ITO 101 does not correspond to only one color of the color filters Rxe2x80xa2Gxe2x80xa2B, but corresponds to two or more adjacent colors of the color filters Rxe2x80xa2Gxe2x80xa2B. FIG. 11(b) is an example where the color mixture defect occurs due to low accuracy of dimension, FIG. 11(c) is an example where the color mixture defect occurs due to low accuracy of position, and FIG. 11(d) is an example where the color mixture defect occurs due to low accuracy of both dimension and position.
Incidentally, a liquid crystal display device, etc., is generally manufactured in such a manner that a plurality of panel patterns are simultaneously formed in a large substrate, and, after undergoing various processes, the large substrate is separated into respective panels. The size of the substrate used is from approximately 300 mm square to not less than 1000 mm square. Thus, an entire surface of the substrate must satisfy the accuracy of dimension and position and the accuracy of superposition, which are required for forming the patterns.
Here, the accuracy of dimension and position for forming a pattern by a photolithography method is determined mainly at an exposing step, so that it largely depends on the performance of a developing device, photomask""s accuracy of dimension and form, the performance of photosensitive resin, etc.
However, when the relative accuracy in superposition of a plurality of patterns is required, as in the above-described liquid crystal display device, required is not only the accuracy of dimension and position of the pattern that is formed at the exposing step. Even if the pattern can achieve high accuracy of dimension at the exposing step, when the substrate itself changes in dimension due to thermal expansion, etc., the dimensions of the already formed pattern also change. This causes a dimensional error between the pattern that is already formed and the pattern that is newly formed, thereby lowering accuracy of superposition between the respective formed patterns. Thus, at the exposing of the newly formed pattern, it is required to control the substrate dimensions so that the dimensions and form of the already formed pattern are retained.
Nowadays, as a substrate for a display device such as a liquid crystal display device, an organic EL display device, an electrophoresis display device, etc., a plastic substrate has come to be used instead of a conventional glass substrate. The plastic substrate has advantages over the glass substrate in terms of lightness in weight, shock resistance, flexibility, etc. However, the plastic substrate widely changes in dimension by a slight temperature change because of its high coefficient of thermal expansion. The plastic substrate also expands due to moisture absorption. As described above, the plastic substrate requires a higher level of technique of dimensional control, because the plastic substrate changes in dimension more than the glass substrate.
In order to control the dimensional change of the substrate due to the thermal expansion, the substrate temperature during the exposing should be controlled, so that general exposing devices have a structure capable of adjusting a substrate temperature. However, in order to achieve the accuracy of dimensional control of the plastic substrate same as that of the glass substrate, the exposing device must have about twice to tenth higher accuracy of adjusting the substrate temperature, depending on concrete materials and compositions of the plastic and the glass.
In order to control the dimensional change of the substrate due to the moisture absorption, it is highly effective to employ a method to control a moisture content of the substrate, such as inhibition of the moisture absorption using a hard-coat layer, dehydration of the substrate by heating, moisture absorption by rinsing, etc., as disclosed in Japanese Unexamined Patent Publication No. 6-186550/1994 (Tokukaihei 6-186550, published on Jul. 8, 1994), for example.
Incidentally, it is conventionally assumed that a dimensional behavior of the plastic substrate mainly depends on xe2x80x9cthe dimensional change due to the thermal expansionxe2x80x9d and xe2x80x9cthe dimensional change due to the moisture absorptionxe2x80x9d; thus, when the pattern is formed by the photolithography method, it is assumed that the dimensional behavior of the plastic substrate can be controlled by xe2x80x9ccontrol of the substrate temperature at the exposingxe2x80x9d and xe2x80x9ccontrol of the moisture content of the substrate at the exposingxe2x80x9d, enabling the pattern formation with high accuracy of dimension and superposition.
However, the above-described pattern forming method has problems as described below.
Specifically, as to the method for the xe2x80x9ccontrol of the moisture content of the substrate at the exposingxe2x80x9d, when the substrate is exposed after heated for reducing the moisture content of the substrate to a certain level, for example, the substrate may be arranged so as to include a moisture barrier layer such as a hard-coat layer, to inhibit the moisture absorption. This can reduce the speed of substrate expansion due to the moisture absorption after the end of the baking, and thus reduces the dimensional variations among the formed patterns due to the variations in exposing timing, thereby achieving the high accuracy of dimensional control.
The barrier layer for inhibiting the moisture absorption of the substrate is excellently an inorganic thin film, for example, and the inorganic thin film having a denser film structure, in particular, has higher barrier properties. However, even the plastic substrate, having the dense inorganic thin film on each of both sides so as to inhibit the moisture absorption of the substrate, gradually absorbs moisture through a long leaving time, so that the plastic substrate expands. Thus, in order to maintain the dimensional accuracy of the substrate, so as to achieve the accuracy in superposition of a plurality of formed patterns, the pattern is presumably formed in a following manner: the moisture content of the plastic substrate, which has the inorganic thin film as the moisture barrier layer on each of both sides so as to reduce the speed of moisture absorption, is reduced to a certain level by heating of the substrate; and the exposing and the pattern formation are rapidly carried out under control of the substrate temperature, so that the dimensional change of the plastic substrate due to the expansion by the moisture absorption is kept within a predetermined range.
However, it turned out that, when the plastic substrate having the dense inorganic thin film was cooled down, after heating, to a certain temperature and left, the substrate dimensions were not gradually expanded by the moisture absorption, but, on the contrary, gradually contracted. In other words, it turned out that the dimensional behavior of the plastic substrate did not solely depend on xe2x80x9cthe dimensional change due to the thermal expansionxe2x80x9d and xe2x80x9cthe dimensional change due to the moisture absorptionxe2x80x9d. Thus, the accuracy in superposition of formed patterns on the plastic substrate having the dense inorganic thin film cannot be achieved by rapidly exposing the plastic substrate after heating, under control of the substrate temperature.
The object of the present invention is to provide a pattern forming method capable of forming a plurality of patterns on a plastic substrate with high accuracy of superposition, and a display device manufactured using the same.
In order to attain the foregoing object, a pattern forming method of the present invention for forming a predetermined pattern on a photosensitive resin film by (i) layering the photosensitive resin film on an inorganic thin film with which a plastic substrate is coated and (ii) exposing the photosensitive resin film via a photomask having the predetermined pattern in an exposing step is characterized by including the step of heating the plastic substrate having the inorganic thin film before the exposing step, a time from an end of the heating step to a start of the exposing step being managed to be not less than a predetermined time, in accordance with an asymptotic contracting behavior after the end of the heating step of the plastic substrate having the inorganic thin film.
Compared with the glass substrate, the plastic substrate having the inorganic thin film requires a higher level of technique of dimensional control, thereby having difficulty in achieving accuracy in pattern formation. As for this explanation, the inventors of the present invention have found that the plastic substrate having the inorganic thin film does not immediately become stable in dimension, but keeps asymptotically contracting, after keeping the substrate temperature constant by cooling the substrate following the heating step.
Thus, the high accuracy of dimensional control of the substrate cannot be achieved directly after the heating step, but can be achieved when the contraction rate of the substrate decreases in accordance with the asymptotic contracting behavior. Thus, with managing a time from the end of the heating step to the start of the exposing step of the substrate to be not less than the predetermined time, a predetermined pattern should be formed on the photosensitive resin by exposing the photosensitive resin film via the photomask having a predetermined pattern in the exposing step. This enables the pattern formation with high accuracy of dimension, thereby achieving high accuracy in superposition of patterns when a plurality of patterns are formed.
As a result, it is possible to provide a pattern forming method capable of forming a plurality of patterns on the plastic substrate.
In order to attain the foregoing object, a display device of the present invention, manufactured using a pattern forming method for forming a predetermined pattern on a photosensitive resin film by (i) layering the photosensitive resin film on an inorganic thin film with which a plastic substrate is coated and (ii) exposing the photosensitive resin film via a photomask having the predetermined pattern in an exposing step, is characterized in that the pattern forming method includes the step of heating the plastic substrate having the inorganic thin film before the exposing step, a time from an end of the heating step to a start of the exposing step being managed to be not less than a predetermined time, in accordance with an asymptotic contracting behavior after the end of the heating step of the plastic substrate having the inorganic thin film.
With this arrangement, the display device is manufactured using a pattern forming method for forming a predetermined pattern on a photosensitive resin film by (i) layering the photosensitive resin film on an inorganic thin film with which a plastic substrate is coated and (ii) exposing the photosensitive resin film via a photomask having the predetermined pattern in an exposing step. Here, the pattern forming method includes the step of heating the plastic substrate having the inorganic thin film before the exposing step, and a time from an end of the heating step to a start of the exposing step is managed to be not less than a predetermined time, in accordance with an asymptotic contracting behavior after the end of the heating step of the plastic substrate having the inorganic thin film. With this, it is possible to easily manufacture a high definition display device using the plastic substrate, such as a liquid crystal display device, an organic EL display device, an electrophoresis display device, etc., which was conventionally difficult to be manufactured.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.